Methods of preparing a crosslinked fiber membrane

ABSTRACT

Disclosed herein is a method for preparing a crosslinked hollow fiber membrane. The method involves spinning a one phase solution comprising a monoesterified polyimide polymer, acetone as a volatile solvent, a spinning solvent, a spinning non-solvent, and optionally an organic and/or inorganic additive, wherein the volatile solvent is present in an amount of greater than 25 wt. % to about 50 wt. %, based on the total weight of the solution.

BACKGROUND

1. Technical Field

The present invention generally relates to methods for preparingcrosslinked fiber membranes and their use in separating components of agaseous mixture.

2. Description of the Related Art

Polymeric membranes for separating mixtures of gases, such as methaneand carbon dioxide are known. For example, U.S. Pat. Nos. 7,247,191;6,932,859; and 6,755,900, disclose crosslinkable polymers andcrosslinked hollow fiber membranes made from such crosslinkablepolymers. These patents further disclose a crosslinkable polyimidepolymer. The crosslinkable polyimide polymer can be made bymonoesterifying a polyimide polymer with a crosslinking agent.

A crosslinked hollow fiber membrane can be made by forming fibers fromthe crosslinkable polyimide polymer and transesterifying thecrosslinkable polyimide polymer within the fibers. More specifically,the crosslinkable polyimide polymer can be formed into crosslinkablefibers, which are then subjected to transesterification conditions inorder to create covalent ester crosslinks within the fibers. Such fiberscan be hollow fibers or other types of fibers. Crosslinked hollow fibermembranes can be incorporated into a separation module. Other types ofmembranes for separation include flat sheet separation membranes or flatstack permeators.

Integrally skinned hollow fiber membranes can be formed by contactingthe polymer solution with a non-solvent and forming the membrane in aone step process. On contact with the non-solvent, mass transfer takesplace between the non-solvent from the coagulation bath and the solventin the nascent membrane resulting in micro-phase separation within themembrane. Depending on the pathway of phase separation, a dense layer,also called the skin layer, is believed to form on the surface of themembrane. The skin formation is hypothesized to occur when solventoutflow from the membrane exceeds the non-solvent inflow resulting indelayed demixing. This process increases the concentration of thepolymer at the membrane-coagulant interface and forms the skin. Anevaporative step in the air gap can be included prior to the phaseseparation step to enhance skin formation by the evaporation of thevolatile solvent from the nascent membrane followed by a rapid phaseseparation of the underlying region to form a highly porous support.

Polymer solutions used in hollow fiber membrane spinning consist ofpolymer, solvents, non-solvent and additives. When the number ofcomponents exceeds three, a pseudo-ternary phase diagram of more thanthree components can be devised by dividing the components intocategories of polymer, solvent and non-solvent. Within each category,the components can be fixed in ratio to each other to restrict solvencyand/or non-solvency power. This approach based on fixed ratios enablesholding solvency parameters constant for the solvents and nonsolventsthat can be explored in the system and a binodal (set of concentrationsseparating the single phase and two phase regions) obtained.

While not wishing to be bound by any particular theory, ternary phasediagrams can be developed (1) by the titration of the polymer solutionwith non-solvent, (2) through the use of the three-phase Flory-Hugginstheory for polymer solutions, and (3) by inspection of polymer solutionsof various compositions of polymer/solvent/nonsolvent. Depending on thepolymer viscosity in solution, the dope compositions are made to coverthe region of interest for fiber spinning (usually 20 to 40 wt. %polymer). The binodal curve can be generated by making small samples (10to 15 gram) of various compositions and visually inspecting them forphase separation.

Once the binodal has been identified, three factors taken intoconsideration when determining the dope formulation are: (1) proximityof the dope composition to the binodal, (2) osmotic pressure of thesolution, and (3) polymer solution viscosity.

The proximity of the polymer solution composition to the binodal andosmotic pressure of the solution determine the kinetics of membraneformation and membrane morphology. Osmotic pressure has earlier beensuggested as the cause for the large finger/tear shaped voids(macrovoids) found in certain membranes. To describe the phaseseparation of the polymer solution (in forming the membrane), a ternarydiagram can be formed which groups all the solvents, nonsolvents andadditives into the solvent category, and depicts the coagulant(typically water) in the nonsolvent category. Based on the proximity ofthe polymer solution to the binodal, the quantity of coagulant requiredto phase separate the polymer solution can be determined. Since thepenetration of the coagulant into the polymer solution is limited by therate of diffusion, the distance of the polymer solution from the binodaland the osmotic pressure driving force determines the rate and type ofphase separation. Compositional change on the ternary phase diagram(FIG. 1) from point 1 (original polymer solution) to point 2 ishypothesized for the skin and from point 1 to an arbitrary position 3(in the spinodal region) for the support layer of the membrane. Theobjective is to drive phase separation of the support layer throughspinodal decomposition mechanism to form a highly porous support withlittle or no resistance to gas flow.

The minimum polymer solution viscosity depends on the strength of thepolymer solution strand which undergoes elongation (under gravity) thattakes place after the fiber exits the spinneret. Based on the air gapand draw ratio, this minimum viscosity must be defined for eachpolymer/solvent/nonsolvent system. A higher viscosity can be achieved byincreasing the polymer concentration in the polymer solution or byadding viscosity enhancers, like lithium nitrate (LiNO₃) and carboxylicacids which complex with the common spinning solvents (i.e.N-methyl-2-pyrrolidone). Although a high polymer concentration isgenerally required to promote skin growth and increase viscosity forspinning, it is believed that too high of a polymer concentration wouldreduce porosity in the support layer and form a support layer withsubstantial resistance to gas flow which is undesirable.

Solvents and non-solvents are selected, in part, for their miscibilitywith the aqueous coagulant. Another factor for consideration in theselection of the polymer solution solvent is the generation of osmoticpressure during phase separation. The osmotic pressure is a function ofthe thermodynamic activities of the solvent and coagulant non-solvent,and is believed to be a factor in the formation of macrovoids.

The crosslinked hollow fiber membranes have good permeability andselectivity. The crosslinked hollow fiber membranes also have goodresistance to plasticization. Plasticization occurs when one or morecomponents of a fluid mixture causes the polymer to swell therebyaltering the properties of the membrane. For example, polyimides areparticularly susceptible to plasticization by carbon dioxide. Subjectingthe fibers to transesterification conditions to crosslink thecrosslinkable polyimide polymer within the fibers increases bothresistance to plasticization and selectivity.

The above referenced patents disclose the use of sufficiently highmolecular weight polyimide polymers to accommodate for molecular weightloss during the monoesterification process. However, it is difficult toproduce crosslinkable polyimide polymers having such a high molecularweight. Therefore, there is a need for a method of making acrosslinkable (i.e., monoesterified) polyimide polymer that reduces oreliminates the loss of molecular weight during the monoesterificationprocess, i.e., a high molecular weight, monoesterified polyimidepolymer, while having improved strength, flexibility, and/orspinnability.

SUMMARY

In accordance with one embodiment, there is provided a method forpreparing a crosslinked hollow fiber membrane, which comprises spinninga one phase solution comprising a monoesterified polyimide polymer,acetone as a volatile solvent, a spinning solvent, and a spinningnon-solvent, wherein the volatile solvent is present in an amount ofgreater than 25 wt. % to about 50 wt. %, based on the total weight ofthe solution.

In accordance with a second embodiment, there is provided a method forpreparing a crosslinked hollow fiber membrane, which comprises spinninga one phase solution comprising a monoesterified polyimide polymer,acetone as a volatile solvent, a spinning solvent, and a spinningnon-solvent, wherein the volatile solvent is present in an amount ofgreater than 25 wt. % to about 50 wt. %, based on the total weight ofthe solution, and subjecting the monoesterified polyimide polymer fiberto transesterification conditions to form a crosslinked polyimide hollowfiber membrane.

In accordance with a third embodiment, there is provided a method forpreparing a crosslinked hollow fiber membrane, which comprises spinninga one phase solution comprising a monoesterified polyimide polymer, avolatile solvent having a threshold limit value time-weighted average(TLV-TWA) toxicity of greater than 200 parts per million (ppm) exposurelimit, a spinning solvent, and a spinning non-solvent, wherein thevolatile solvent is present in an amount of greater than 25 wt. % toabout 50 wt. %, based on the total weight of the solution.

The use of a relatively high concentration of acetone in the polymersolution in place of tetrahydrofuran (THF) advantageously obtains asufficient quantity of evaporative solvent (e.g., acetone) while alsomaintaining a single phase polymer solution. Acetone has less stringentstorage requirements compared to THF, which can form explosiveperoxides. In addition, while not wishing to be bound by theory, it isbelieved that the use of a relatively high concentration of acetone inthe polymer solution aids in the skin formation of the membrane in theair gap resulting in less skin defects. It is further believed that theuse of a relatively high concentration of acetone in the polymersolution aids in limiting the skin formation in the coagulant bathduring membrane production and hastening phase separation to form a moreporous support with a minimal transition layer and relatively defectfree skin in the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the compositional change on a ternary phase diagram.

FIG. 2 shows both a monesterification reaction and a transesterificationreaction.

FIG. 3 is a schematic representation of an asymmetric bilayer and agraded density skin asymmetric trilayer.

DETAILED DESCRIPTION

The present invention is directed to methods for preparing an integrallyskinned, gas permeable asymmetric crosslinked hollow fiber. In general,the methods involve at least spinning a one phase solution comprising amonoesterified polyimide polymer, acetone as a volatile solvent, aspinning solvent, a spinning non-solvent, and optionally an organic orinorganic additive, wherein the volatile solvent is present in an amountof greater than 25 wt. % to about 50 wt. %, based on the total weight ofthe solution.

Definitions

The following terms are used throughout the specification and have thefollowing meanings unless otherwise indicated.

As used herein, the term “carboxylic acid functional group” refers to apendant group of —COOH—.

The term “diol” refers to a chemical compound containing two hydroxylgroups.

The term “carbodiimide” means a chemical compound containing thefunctional group N═C═N.

The term “dianhydride” refers to any compound that contains twoanhydride

groups.

The term “halogenated alkyl” means a straight-chain or branchedsaturated monovalent hydrocarbon group of one to twelve carbon atoms,wherein at least one of the carbon atoms is replaced by a halogen atom(e.g. fluoromethyl, 1-bromo-ethyl, 2-chloro-pentyl, 6-iodo-hexyl, andthe like).

The term “halo” or “halogenated” refers to a functional group includinga halogen atom such as fluorine, chlorine, bromine, or iodine.

The term “phenyl” means an aromatic group of six carbon atoms having theformula —C₆H₅.

The term “alkyl” means a straight-chain or branched saturated monovalenthydrocarbon group of one to twelve carbon atoms (e.g. methyl, ethyl,i-propyl, and the like). Alkyl groups have the formula C_(n)H_(2n+1)where n is a positive non-zero integer.

The term “diamino cyclic compound” means a chemical compound having aring structure of three to twelve carbon atoms where the ring structureis functionalized by two amino or substituted amino groups.

The term “amino” means a functional group having the formula —NR′R″where R′ and R″ are independently H, alkyl, cycloalkyl, and aryl.

The term “cycloalkyl” means a cyclic saturated monovalent hydrocarbongroup containing 3 to 12 carbon atoms having a single cyclic ring ormultiple condensed rings. Such cycloalkyl groups include, by way ofexample, cyclopropyl, cyclohexyl, cyclooctyl, adamantanyl, and the like.

The term “aliphatic” refers to non-aromatic organic compounds, in whichcarbon atoms are joined together in straight or branched chains.Aliphatic includes paraffinic (e.g., alkyl), olefinic (e.g., alkenyl),and alkynyl compounds.

The term “antilyotropic salt” refers to a salt that interacts withsolvent molecules rather than polymer molecules.

The term “amide” means a functional group having a carbonyl group (C═O)linked to a nitrogen atom or a compound that includes this functionalgroup.

The term “ester” means a functional group having a carbonyl group (C═O)linked to an alkoxy group.

The term “alkoxy” refers to an alkyl group linked to an oxygen atom suchas, for example, methoxy or ethoxy.

The term “aryl” refers to an unsaturated aromatic carbocyclic group offrom 6 to 20 carbon atoms having a single ring (e.g., phenyl) ormultiple condensed (fused) rings (e.g., naphthyl or anthryl). Exemplaryaryls include phenyl, naphthyl and the like.

The term “alkenyl” refers to a linear or branched unsaturated monovalenthydrocarbon group having 2 to 12 carbon atoms and containing at leastone, for example, from 1 to 3 double bond(s). This term is exemplifiedby groups such as ethenyl, 2-propenyl, and the like.

The term “alkynyl” refers to a linear or branched monovalent hydrocarbongroup having 2 to 12 carbon atoms and containing at least one, forexample, from 1 to 3 triple bond(s). This term is exemplified by groupssuch as ethynyl, 2-propynyl, n-butynyl and the like.

As used herein, the term “reduce” means to decrease or diminish.

Whenever used herein, the term “molecular weight” or “average molecularweight” means weight average molecular weight as measured by GelPermeation Chromatography (GPC) using polystyrene as the standard. Thismethod is described in ASTM D5296-05.

“Draw ratio” refers to the ratio of the take-up rate of an extrudedfiber to the extrusion rate of the fiber.

I. Method of Making Monoesterified Polyimide Polymer

The monoesterified polyimide polymer can be obtained by (a) preparing apolyimide polymer comprising carboxylic acid functional groups from areaction solution comprising monomers and at least one solvent; and (b)treating the polyimide polymer with a diol at esterification conditionsin the presence of dehydrating conditions to form a monoesterifiedpolyimide polymer, wherein the dehydrating conditions at least partiallyremove water produced during steps (a) and (b).

Step (a)-Polymerization Reaction and Imidization Reaction

In step (a), monomers are polymerized to form a polyamide polymercomprising amide bonds. Next, in step (a), an imidization reactionoccurs wherein the amide bonds of the polyamide polymer form imide bondstransforming the polyamide polymer into a polyimide polymer and productwater is removed. The resultant polyimide polymer includes carboxylicacid functional groups which are capable of crosslinking chains of thepolyimide polymer.

Monomers

The monomers can comprise between about 15 and about 25 weight percentof the reaction solution.

At least some of the monomers include carboxylic acid functional groupssuch that the resultant polyimide polymer comprises carboxylic acidfunctional groups. The monomers can include dianhydrides,tetracarboxylic acids, and furandiones. The monomers can further includediamino compounds such as diamino cyclic compounds and diaminoaromatics. The diamino aromatics can have more than one aromatic ringwhere the amino groups are on the same or different aromatic ring.

For example, the monomers can include monomers A, B, and C wherein A isa dianhydride of formula (I):

X₁ and X₂ are independently selected from halogenated alkyl, phenyl orhalogen;

R₁, R₂, R₃, R₄, R₅, and R₆ are H, alkyl, or halogen;

B is a diamino cyclic compound without a carboxylic acid functionality;and

C is a diamino cyclic compound with a carboxylic acid functionality.

If the monomers are comprised of the monomers A, B, and C, the ratio ofB to C can be between 1:4 and 8:1.

The monomer A can be 4,4′-(hexafluoroisopropylidene) diphthalicanhydride (6FDA), which is also known as (2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane. 6FDA has the following formula:

Including 6FDA in the monomers provides stability to the polyimidepolymer because 6FDA has limited rotational ability.

Monomers with limited rotational ability, like 6FDA, are desirablebecause they increase the selectivity of the membrane made according tothe methods disclosed herein. Monomers with bulky side groups, like(CF₃)₂ in 6 FDA, also inhibit chain packing, which increasespermeability of molecules through the membrane. Both selectivity andpermeability are important for efficient and productive separations.Further reference to these structure property relationships can be foundin Koros and Fleming, Journal of Membrane Science, 83, 1-80 (1993).

The monomer B, a diamino cyclic compound without a carboxylic acidfunctionality, can be a diamino aromatic compound with more than onearomatic ring where the amino groups are on the same or differentaromatic rings. For example, the monomer B can be 4,4′ isopropylidenedianiline, 3,3′ hexafluoroisopropylidene dianiline, 4,4′hexafluoroisopropyliene dianiline, 4,4′ oxydianiline, 3,3′ oxydianiline,4,4′ diaminodiphenyl, diaminotoluene, diaminobenzotrifluoride,dimethyldiaminobenzene, trimethyldiaminobenezene, ortetramethyldiaminobenzene. The monomer B can also be2,4,6-trimethyl-m-phenylenediamine (DAM), which is represented by thefollowing formula:

The monomer C, a diamino cyclic compound with a carboxylic acidfunctionality, can be diamino benzoic acid. It is represented by thefollowing formula:

More specifically, the monomer C can be 3,5 diaminobenzoic acid (DABA).

In one embodiment of the methods as described herein, the monomersinclude A, B, and C where A is 6FDA, B is DAM, and C is DABA. In thisembodiment, the 6FDA content of the monomer mixture is about 50 percentand the remaining about 50 percent of the monomer mixture is composed ofDAM and DABA. The DABA content is between about 20 percent and about 100percent of the remaining about 50 weight percent. For example, the 6FDAcontent of the monomer mixture can be about 50 percent and the remainingabout 50 percent can be about 40 percent DABA and about 60 percent DAM.When 6FDA, DAM, and DABA are present in these stoichiometricconcentrations, the polyimide polymer formed in step (a) is representedby the formula (II):

In another embodiment of the methods as described herein, the monomersinclude A, B, and C where A is 6FDA, B is DAM, and C is DABA as well asone or more additional dianhydrides.

Whichever monomers are used, according to some embodiments as describedherein, they can be purified prior to step (a). The monomers can bepurified by techniques known in the art, for example, sublimation orrecrystallization.

Solvents

The monomers are dissolved in at least one solvent to create a reactionsolution and facilitate polymerization. The resulting polyamide polymerremains in the reaction solution for imidization. The at least onesolvent can comprise between about 75 and about 95 weight percent of thereaction solution. The at least one solvent can be at least one highboiling organic solvent. The solvent can also be mixtures of organicsolvents. Exemplary high boiling organic solvents are listed in Table 1along with their normal boiling points.

TABLE 1 High boiling organic solvent Normal boiling point (° C.)N-Methyl-2-pyrrolidone (NMP) 202.1 Dimethyl sulfoxide (DMSO) 190Dimethylformamide (DMF) 152.9 Dimethylacetamide (DMAc) 165.1 Diglyme 162

Accordingly, the solvent of the reaction solution can be any one of theorganic solvents listed above or mixtures thereof. High boiling solventsare desirable because they prevent excessive evaporation, which wouldsignificantly alter concentrations in the reaction solution andconcentrations during subsequent processing.

Dehydrating Conditions

If dehydrating conditions are utilized during step (a) to remove water,the concentration of water in the reaction solution can be maintained atbetween about 0 weight percent and about 0.26 weight percent.

The dehydrating conditions can be the presence of a chemical dehydratingagent and/or a mechanical dehydrating agent. The dehydrating conditionscan be the presence of a chemical dehydrating agent only, a mechanicaldehydrating agent only, or the combination of a chemical dehydratingagent and a mechanical dehydrating agent.

If a chemical dehydrating agent is utilized, the chemical dehydratingagent does not impede the imidization reaction of step (a). For example,it does not decrease the imidization reaction rate or decrease themonoesterified, polyimide polymer yield. The chemical dehydrating agentcan form an azeotrope with water, which can be boiled out of thereaction solution. Such azeotropic chemical dehydrating agents are wellknown to one of ordinary skill in the art. Exemplary azeotropic chemicaldehydrating agents include ortho-dichlorobenzene (ODCB), benzene,toluene, and mixtures thereof. Alternatively, the chemical dehydratingagent can be a carbodiimide.

If an azeotropic chemical dehydrating agent is used as the chemicaldehydrating agent, it can be used in relatively large amounts, forexample, between about 1 ml and about 4 ml per gram of the polyamidepolymer. Such a large amount of azeotropic chemical dehydrating agentensures that the water produced by the imidization reaction is removedfrom the reaction solution.

If a carbodiimide is used as the chemical dehydrating agent, it can beused in an amount between about 1 and about 4 times the stoichiometricamount based on moles of water removed.

The chemical dehydrating agent can also be periodically added to thereaction solution throughout step (a). For example, ODCB can be addedperiodically. According to one embodiment of the method as describedherein, the chemical dehydrating agent is added to the reaction solutionin three separate batches.

If a mechanical dehydrating agent is utilized, the mechanicaldehydrating agent is a physical system designed to remove water. Anexemplary mechanical dehydrating agent is a Dean-Stark trap. Dean-Starktraps are well known to those of ordinary skill in the art. Anymechanical system that prevents water distilled from the reactionsolution from returning to the reaction solution can be suitable.

Polymerization Conditions

In the polymerization reaction of step (a), monomers are polymerized inthe reaction solution to form a polyamide polymer. Polymerization canoccur at room temperature while the reaction solution is stirred orotherwise agitated. Solvent concentration during polymerization isbetween about 75 and about 95 weight percent of the reaction solution.

Imidization Conditions

In the imidization reaction of step (a), the amide bonds of thepolyamide polymer form imide rings to provide the polyimide polymer. Theimidization reaction in step (a) occurs over an extended period of time,about 12 to about 36 hours. Such an extended period of time ensures thatthe imidization reaction proceeds to completion, which is important withrespect to yield of the polyimide polymer. The imidization reaction canoccur at temperatures between about 160° C. and about 200° C. Solventconcentration during imidization is between about 75 and about 95 weightpercent of the reaction solution.

Step (b)—Monoesterification Reaction

Step (b) involves treating the polyimide polymer with a diol atesterification conditions in the presence of the dehydrating conditionsto form a monoesterified polyimide polymer. Thus, during step (b), thepolyimide polymer is subjected to monoesterification. After theimidization reaction of step (a) is complete, the reaction solutioncomprises the polyimide polymer, the at least one solvent, and anyunreacted monomers. The diol can be directly added to the reactionsolution as a crosslinking agent to form a monoesterification reactionsolution. Thus, both the imidization reaction of step (a) and themonoesterification reaction of step (b) can take place in one reactionvessel or “one pot”. Alternatively, the polyimide polymer can beisolated and then combined with the diol to form a monoesterificationreaction solution such that the imidization reaction of step (a) and themonoesterification reaction of step (b) take place in separate reactionvessels.

FIG. 2 schematically illustrates the monoesterification reaction. Asexplained above, the monoesterification reaction involves one of the —OHgroups in the diol molecules reacting with the —COOH groups of thepolyimide polymer to convert the —COOH groups to esters and provide themonoesterified polyimide polymer. Water is also produced as a by-productduring monoesterification. Importantly, in the method as describedherein, at least a portion of the water is removed from themonoesterification reaction solution by the dehydrating conditions.

Along with the diol, an acid catalyst can also be added to the reactionsolution to facilitate the monoesterification reaction.

The monoesterified polyimide polymer produced by step (b) can have anaverage molecular weight between about 50,000 and 300,000. In oneembodiment, the monoesterified polyimide polymer has an averagemolecular weight between about 100,000 and about 200,000. In anotherembodiment, the monoesterified polyimide polymer has an averagemolecular weight between about 125,000 and about 200,000. Themonoesterified polyimide polymer can also have a polydispersity indexbetween about 2 and about 5.

In step (b), a monoesterification reaction takes place. Morespecifically, the carboxylic acid functional groups (—COOH) of thepolyimide polymer react with the hydroxyl functional groups (—OH) of thediol to convert the —COOH groups to esters. This provides amonoesterified polyimide polymer and water as a by-product. Each diolmolecule contains two —OH groups. During monoesterification, only one ofthe —OH groups of each diol molecule reacts with a —COOH group. Ideally,the conversion of —COOH groups to esters (i.e., the ester yield) isalmost 100%. However, in some cases, the ester yield can be less than100%. Any unconverted —COOH groups can act as crosslinkable sites in alater transesterification reaction whereby monoesterified polyimidepolymer chains are crosslinked.

Moreover, in step (b), dehydrating conditions at least partially removethe water by-product such that the average molecular weight of themonoesterified polyimide polymer is partially maintained, fullymaintained, or even increased. By at least partially removing thewater-byproduct, which is only present in very small amounts, molecularweight retention during the monoesterification reaction to a significantdegree is affected. While not wishing to be bound by any particulartheory, it is believed that water can attack the imide rings of thepolyimide polymer, which can cause chain scissioning and consequentlyreduce the average molecular weight of the polyimide polymer. Theselower molecular weight polyimide polymer chains are then monoesterifiedresulting in a monoesterified, polyimide polymer lower in molecularweight than the original polyimide polymer. Up to about a 70% loss inmolecular weight has been observed during monoesterification absentwater removal. However, when dehydrating conditions are utilized, asdescribed herein to eliminate at least some of the minimal amount ofwater present, a large molecular weight loss is not observed and amolecular weight gain has been obtained in certain instances.

While removal of the minimal amount of water produced duringmonoesterification may to some degree drive the monoesterificationreaction forward, the removal of water is associated with smallermolecular weight loss, maintenance of molecular weight or even molecularweight gain.

Diol

The length of the diol plays a role in forming the monoesterifiedpolyimide polymer of step (b). If the diol is too long or too short, itcan decrease the permeability and/or selectivity of a membrane formedfrom the monoesterified, polyimide polymer. Diols useful in the methodas described herein include ethylene glycol, propylene glycol, 1,3propanediol, 1,4 butanediol, 1,2 butanediol, benzenedimethanol, 1,3butanediol, and mixtures thereof. In one embodiment, the diol isselected from the group consisting of ethylene glycol, propylene glycol,1,3 propanediol, benzenedimethanol, and mixtures thereof. In anotherembodiment, the diol is selected from the group consisting of ethyleneglycol, propylene glycol, 1,3 propanediol, and mixtures thereof. In yetanother embodiment, the diol is selected from the group consisting ofethylene glycol, 1,3 propanediol, and mixtures thereof. In still anotherembodiment, the diol is 1,3 propanediol.

Dehydrating Conditions

As with the optional dehydrating conditions of step (a), the dehydratingconditions of step (b) can result from a chemical dehydrating agentand/or a mechanical dehydrating agent. Therefore, the dehydratingconditions can be a chemical dehydrating agent alone, a mechanicaldehydrating agent alone, or the combination of a chemical dehydratingagent and a mechanical dehydrating agent. It is desirable that thedehydrating conditions, whether chemical or mechanical, remove waterproduced during step (b) from the monoesterification reaction solutionsuch that the concentration of water in the monoesterification reactionsolution is maintained at between about 0 weight percent and about 0.08weight percent.

If a chemical dehydrating agent is utilized, the chemical dehydratingagent does not impede the monoesterification reaction of step (b). Forexample, it does not decrease the monoesterification reaction rate ordecrease the monoesterified, polyimide polymer yield. The chemicaldehydrating agent can be an azeotropic chemical dehydrating agent or canbe a carbodiimide. An azeotropic chemical dehydrating agent forms anazeotrope with the water by-product, which can be boiled out of themonoesterification reaction solution. Such azeotropic chemicaldehydrating agents are well known to those of ordinary skill in the artand include ODCB, benzene, toluene, and mixtures thereof.

A carbodiimide functions as a chemical dehydrating agent byparticipating in the monoesterification reaction by activating thecarboxylic acid functionality of the polyimide polymer toward esterformation and thereby eliminating the water by-product at the same time.This carbodiimide dehydration reaction mechanism is depicted below:

If an azeotropic chemical dehydrating agent is used as the chemicaldehydrating agent, it can be used in relatively large amounts, forexample, between about 1 ml to about 4 ml per gram polyimide polymer.Such a large amount of azeotropic chemical dehydrating agent ensuresthat the water produced by the monoesterification reaction is removedfrom the monoesterification reaction solution.

If a carbodiimide is used as the chemical dehydrating agent, it can beused in an amount between about 1 and about 4 times the stoichiometricamount based on the moles of water removed.

The chemical dehydrating agent can also be periodically added to themonoesterification reaction solution throughout step (b). For example,ODCB can be added periodically. According to one embodiment of themethod as described herein, the chemical dehydrating agent is added tothe monoesterification reaction solution in three separate batches.

As in step (a), the mechanical dehydrating agent is a physical systemdesigned to remove water. An exemplary mechanical dehydrating agent is aDean-Stark trap. Dean-Stark traps are well known to those of ordinaryskill in the art. Any mechanical system that prevents water distilledfrom the monoesterification reaction solution from returning to themonoesterification reaction solution is suitable.

If dehydrating conditions are utilized in step (a), the dehydratingconditions of step (b) can be the same as the dehydrating conditions ofstep (a). In fact, it is desirable for the dehydrating conditions to bethe same because this simplifies the overall method as described herein.In conventional polymerization/imidization/monoesterification reactionmethods, the polyimide polymer is precipitated out of the reactionsolution. However, this extra precipitation step is eliminated when thesame dehydrating conditions are utilized during monoesterification.Further, dehydrating conditions remaining from the imidization reactionof step (a) can be employed in the monoesterification reaction of step(b).

Acid Catalyst

Acid catalysts useful in monoesterification reactions are well known tothose of skill in the art. Acid catalysts activate the carboxylfunctional groups of the polyimide polymer so that they will react withthe hydroxyl groups of the diol. Acid catalysts replace acid chloridesas carboxyl functional group activators. The use of acid chlorides ascarboxyl functional group activators is set forth in Example 1 of U.S.Pat. No. 6,755,900, which is incorporated by reference in its entiretyherein. Exemplary acid catalysts include para-toluene sulfonic acid,sulfuric acid, methanesulfonic acid, triflic acid, and mixtures thereof.If the dehydrating conditions utilized include a carbodiimide, acidcatalyst may not be necessary because the carboxyl functional group ofthe polyimide polymer is activated by the carbodiimide.

The amount of acid catalyst present during the monoesterificationreaction, under dehydrating conditions, also effects the averagemolecular weight of the monoesterified, polyimide polymer. Moreparticularly, it has been discovered that when the amount of acidcatalyst used is less than the conventional amount and dehydratingconditions are present, significantly less molecular weight loss, nomolecular weight loss, or even molecular weight gain, occurs. While notwishing to be bound by any particular theory, it is believed that excessacid catalyst augments degradation of the imide rings of the polyimidepolymer, which causes undesirable chain scissioning and loss of averagemolecular weight. If DABA monomers are used in the method as describedherein, the amount of acid catalyst can be further reduced from theconventional amount. This is due to the fact that DABA monomers areintrinsically acidic.

The acid catalyst can be added in amount ranging from about 0 milligramsto about 0.25 milligrams to the monoesterification reaction solution pergram of the polyimide polymer without experiencing undesirable molecularweight loss.

Monoesterification Conditions

The monoesterification reaction solution, with or without catalyst, isheated to a relatively high temperature over an extended period of time.Generally, the monoesterification reaction solution is heated forapproximately 12 to 30 hours at a temperature between about 120° C. andabout 140° C.

In small (volume) scale reactions, the dehydrating conditions can removewater more easily than in large (volume) scale reactions because thesurface area to volume ratio of the reaction vessel is higher. Such ahigher ratio facilitates boiling of the water.

In large (volume) scale reactions, it is advantageous for both theimidization reaction of step (a) and the monoesterification reaction ofstep (b) to occur in the same reaction vessel. Then any dehydratingconditions remaining from the imidization reaction can be easilyutilized during the monoesterification reaction.

II. Crosslinked Hollow Fiber Membranes: Formation of MonoesterifiedFiber

The method for forming crosslinked hollow fiber membranes involvesforming monoesterified hollow fiber from the monoesterified polyimidepolymer. Because the monoesterified polyimide polymer has a high averagemolecular weight, the monoesterified hollow fiber formed from suchpolymer exhibits increased strength and flexibility. If themonoesterified polyimide polymer is spun into monoesterified hollowfibers, such increased strength and flexibility allow the polymer fibersto be spun at higher take-up rates.

To make such monoesterified hollow fibers, the monoesterified polyimidepolymer can be incorporated into a spinning dope, which is spun intomonoesterified hollow fibers by means of a spinning process such as awet-quench/dry-jet spinning process. While a wet-quench/dry-jet spinningprocess is discussed in detail below, it should be understood that othertypes of spinning methods such as, for example, wet spinning, can beused to form the monoesterified hollow fibers.

Spinning Dope to Form Monoesterified Hollow Fibers

The spinning dope is a homogeneous one phase solution and includes atleast the monoesterified polyimide polymer, a volatile solvent, aspinning solvent, a spinning non-solvent, and optional inorganicadditives.

Sufficient polymer must be present in order to form strong fibers andmembranes capable of withstanding high pressures. However, too muchpolymer increases resistance in the membrane substructure and adverselyaffects membrane performance. In one embodiment of the methods asdescribed herein, the monoesterified polyimide polymer is present in thespinning dope in an amount between about 20 and about 50 weight percent.In another embodiment, the monoesterified polyimide polymer is presentin the spinning dope in an amount between about 25 and about 45 weightpercent. In yet another embodiment, the monoesterified polyimide polymeris present in the spinning dope in an amount between about 30 and about40 weight percent.

In one embodiment, the volatile solvent is acetone. In anotherembodiment, the volatile solvent for use herein should be of asufficiently low toxicity, reported as Threshold Limit Value-TimeWeighted Average toxicity, (TLV-TWA toxicity). This is sometimesvariously referred to only as “TLV” or “TWA” (toxicity). The volatilesolvent for use herein should have a toxicity that is low enough toallow eight hours continuous human exposure and a TWA short-termexposure limit (STEL) over a 15-minute period without adverse effects.The exposure limit, or toxicity, of the volatile solvent is ofimportance to protect the health and well-being of personnel using thematerial. Various government and industrial organizations expresstoxicity in different ways. The Occupational Safety and HealthAdministration (OSHA) expresses toxicity in terms of TLV-TWA which isthe concentration of vapor in parts per million parts of air to whichperson can be exposed for eight hours per day or a 40 hour work weekwithout adverse effects. OSHA also expresses toxicity in terms ofTWA-STEL which is the maximum 15-minute concentration of vapor in partsper million parts of air to which workers may be exposed during any15-minute period of the working day without adverse effects.

Accordingly, in one embodiment, the volatile solvent can be any organicsolvent having a TLV-TWA toxicity of greater than 200 ppm exposurelimit. In another embodiment, the volatile solvent for use herein can beany organic solvent having an OSHA personnel exposure limit greater than200 ppm as an 8-hour TWA concentration and a TWA STEL greater than 250ppm over a 15-minute period. Exemplary volatile solvents of thisembodiment include acetone and the like.

In one embodiment, the volatile solvent is present in the spinning dopein an amount greater than 25 to about 50 weight percent, based on thetotal weight of the spinning dope. In another embodiment, the volatilesolvent is present in the spinning dope in an amount greater than 25 andabout 35 weight percent. In yet another embodiment, the volatile solventis present in the spinning dope in an amount between about 35 and about50 weight percent.

The use of such an organic solvent in relatively high concentrations isbelieved to aid in the formation of the relatively defect free denseskin layer of the hollow fiber through evaporation in the air gap. Theterm “relatively defect free skin” as used herein shall be understood tomean a skinned membranehaving 90% and above permselectivity of its densefilm permselectivity. Moreover, by using relatively high concentrationsof the volatile solvent, a relatively thin transition layer between theporous substructure and relatively defect free dense skin of uniformdensity is believed to be formed (See FIG. 3). This is in contrast to abilayer. This analogy is illustrated in FIG. 3 which presents aschematic representation of bilayer asymmetric and trilayer asymmetricgraded density skin membranes.

Essentially, the transition layer is the layer between the dense skinand the porous support that has gas flow resistance. A transition layercan be determined by a decrease in the He/N₂ permselectivity (i.e.,ideal selectivity; ratio of the two permeabilities or permeances) versusideal O₂/N₂ permselectivity for a hollow fiber when comparing againstintrinsic dense film permselectivities. A hollow fiber membrane having atransition layer with gas resistance will show substantially the sameO₂/N₂ permselectivity for both the fiber and dense film, but a lowerHe/N₂ permselectivity for the fiber. This is due to helium being a“fast” gas which is more affected by the resistance to the transitionlayer than oxygen. The presence of a thick transition layer cantherefore be determined if the He/N₂ permselectivity is about 10% ormore lower than the intrinsic permselectivity (i.e., the dense filmvalue) with the O₂/N₂ permselectivity being essentially the same as thedense film value.

The volatile solvent and/or non-solvent may also effectively andefficiently evaporate during the dry jet step of the dry-jet/wet-quenchspinning process and evaporation on the outside of the nascent membranefiber is believed to help keep the polymer chains more entangled and ata higher concentration, which promotes vitrification and formation ofthe dense skin. The specified room temperature vapor pressure of theorganic solvent can be greater than about 0.05 bar (5 kPa).Alternatively, the specified room temperature vapor pressure can begreater than about 0.1 bar (10 kPa). As another alternative, thespecified room temperature vapor pressure can be greater than about 0.2bar (20 kPa). The specified boiling point of the organic solvent can bebetween about 30° C. and about 100° C. Alternatively, the specifiedboiling point can be between about 40° C. and about 90° C. As anotheralternative, the specified boiling point can be between about 50° C. andabout 70° C.

The optional organic or inorganic additive can enhance phase separation,increase substructure porosity, and increase viscosity of the spinningdope. Since the monoesterified, polyimide polymer has a large quantityof carboxyl functional groups, it is more hydrophilic than mosttraditional polymers used in spinning processes. Therefore, it takes alonger time for the monoesterified polyimide polymer to separate duringthe wet-quench step. The optional inorganic additive reduces the timenecessary for phase separation of the monoesterified polyimide polymer.

The optional inorganic additive can be an antilyotropic salt. The term“antilyotropic salt” as used herein refers to a salt that interacts withsolvent molecules rather than polymer molecules. See Ekiner O. M. etal., Journal of Membrane Science 53 (1990) 259-273. Exemplaryantilyotropic salts include LiNO₃, LiClO₄, MgCl₂, ZnCl₂, and NaI.

While the inorganic additive can reduce the time required for phaseseparation, it is believed that excess inorganic additive (e.g. LiNO₃)can cause defect formation if the porosity extends into thenon-vitrified skin layer of the hollow fiber. In one embodiment, theconcentration of antilyotropic salt in the spinning dope is betweenabout 0 and about 10 weight percent. In another embodiment, theconcentration of the antilyotropic salt in the spinning dope is betweenabout 2 and about 8 weight percent. In yet another embodiment, theconcentration of the antilyotropic salt in the spinning dope is betweenabout 4 and about 7 weight percent.

The spinning solvent can be a high boiling organic solvent. Exemplaryhigh boiling organic solvents are listed in Table 1 above, along withtheir normal boiling points. A high boiling organic solvent that has ahigh affinity for water can enhance phase separation of the hollow fiberin the wet-quench step of the spinning process. N-Methyl-2-pyrrolidione(NMP) is a particularly desirable spinning solvent because it dissolvesmany polymers used in spinning, is relatively benign compared to otherspinning solvents, and has a high affinity for water. The concentrationof the spinning solvent can be dependent upon many factors, includingthe molecular weight of the monoesterified polyimide polymer, thepolydispersity index of the monoesterified polyimide polymer, and theother components of the spinning dope, and can be determined by theprecipitation method discussed below.

The spinning non-solvent can be a C₂ to C₁₀ alcohol, such as analiphatic alcohol, or water. In one embodiment of the methods asdescribed herein, the spinning non-solvent is a lower boiling C₂aliphatic alcohol, for example, ethanol. The normal boiling point ofethanol is 78.4° C. Some spinning non-solvents (e.g., ethanol) can alsoserve as an additional volatile component. The concentration of thespinning non-solvent is directly dependent upon the spinning solventconcentration and can also be determined by the precipitation methoddiscussed below.

The concentrations of spinning solvent and spinning non-solvent can bedetermined by an iterative precipitation method wherein theconcentrations of the spinning solvent and the spinning non-solvent aredependent upon the respective concentrations of the monoesterifiedpolyimide polymer, the volatile component, and the optional inorganicadditive. Such precipitation method ensures that the spinning dope is ahomogeneous one-phase solution, but is still close to the point ofprecipitation in order to reduce the phase separation time during thewet-quench step.

According to the precipitation method, the concentrations of themonoesterified polyimide polymer, the volatile component, and theoptional inorganic additive are set. Initial concentrations of thespinning solvent and the spinning non-solvent are then chosen. Thecomponents, in these concentrations, are combined in a small samplevial. First, the volatile component, the spinning solvent, and thespinning non-solvent are mixed to form a solution. Next, the optionalinorganic additive is added to the solution. After the optionalinorganic additive dissolves in the solution, the monoesterifiedpolyimide polymer is added to the solution to provide a spinning dopesample. The polymer can be added in batches to facilitate dispersion ofthe polymer throughout the solution. If the polymer precipitates out,the spinning solvent concentration is increased anywhere between about 0weight percent and about 5 weight percent to arrive at the finalspinning solvent concentration. The spinning non-solvent concentrationis similarly decreased to arrive at the final spinning non-solventconcentration. If the polymer does not precipitate out, theconcentration of the spinning solvent and/or the spinning non-solvent isaltered and the precipitation test is repeated. Iterations occur untilfinal concentrations are obtained that provide a homogeneous one-phasespinning dope close to the point of precipitation.

A larger amount of spinning dope can be prepared according to thesefinal concentrations. It is advantageous to carry out the precipitationmethod with small sample amounts of spinning dope before spinning anybatch of the spinning dope because the point of precipitation can varyas the structure and/or average molecular weight of the polymer varies.

Dry-Jet/Wet-Quench Spinning Process to Form Monoesterified Hollow Fibers

If a dry-jet/wet-quench spinning process is used to spin the highmolecular weight, monoesterified polyimide polymer into hollow fibers,the skin and porous support layer can be formed in a single process.

Dry-jet/wet-quench spinning processes are well known in the art.Generally, in a dry-jet/wet-quench spinning process, spinning dopecomprising a polymer is extruded into fibers or filaments throughorifices of a spinneret, which is separated from a coagulating bath by agaseous layer or non-coagulating liquid. The filaments are passedthrough the gaseous layer, such as air, or non-coagulating liquid, suchas toluene or heptane, and then conducted into a coagulating bath.Conveyance of the filaments through the gaseous layer is commonlyreferred to as the dry jet step. The coagulating bath can be an eitheran aqueous system, such as pure water, or a non-aqueous system, such asmethanol. Conveyance of the filaments through the coagulating bath iscommonly referred to as the wet-quench step. After the filaments leavethe coagulating bath, they can be washed. Washing is especiallyimportant if the coagulating bath contains any acid and can beaccomplished with water alone or combinations of alkaline solutions andwater. The filaments are dried and wound on a rotating drum. They can beair dried on the drum or the drum can be heated to facilitate drying.

According to an embodiment of the method of making the crosslinkedhollow fiber membrane as described herein, a monoesterified polyimidepolymer is extruded through orifices of a spinneret to provide hollowfibers. These hollow fibers are conveyed through a gaseous layer of airand through a coagulating bath of de-ionized water. The fibers exit thede-ionized water bath and are wound around a take-up drum.

The take-up drum can be partially contained in a vessel of roomtemperature de-ionized water in order to keep the fibers wet. The fiberscan be left on the take-up drum for between about 10 minutes and about20 minutes and then cut into strands and left in another de-ionizedwater bath for between about 2 days and about 3 days. The de-ionizedwater baths help remove solvent from the fibers. Water from the fiberscan then be removed by fluid exchange with non-solvents of decreasingsurface tension, for example, ethanol followed by removal of ethanol byhexane. Ultimately, the fibers can be air-dried and/or oven-dried.

According to the method as described herein, the spinneret orifices canhave smaller dimensions than those used in conventional spinningprocesses. Smaller spinneret dimensions permit spinning of hollow fibersunder normal conditions into fibers useful for making membranes that canbe used under high pressure conditions, i.e., fibers with a diameter ofless than 300 microns. The smaller spinneret dimensions also improvemixing in the spinneret and shearing during extrusion. Further, thesmaller spinneret dimensions increase the extrusion velocity andconsequently decrease the draw ratio, i.e., the take-up rate divided bythe extrusion rate. Reduced draw ratios are desirable becauseexcessively high draw ratios can induce high orientation/elongationstresses, which may be detrimental during further processing likecrosslinking. For example, it was found that when hollow fibers werespun with a spinneret having larger dimensions, high draw ratios had tobe applied to achieve fibers of reasonable dimensions (less than 300microns) and these fibers became defective after crosslinking.

The annular diameter of the spinneret orifices can be approximately halfthe size of conventional spinneret orifices. For example, the annulardiameter can be between about 600 microns and about 1300 microns and thebore needle outer diameter can be between about 300 microns and about700 microns.

The draw ratio can be less than 150. Alternatively, the draw ratio canbe less than 100. As another alternative, the draw ratio can be lessthan 50. As still another alternative, the draw ratio can be less than10.

The distance between the point of extrusion out of the spinneret and thesurface of the de-ionized water bath is referred to herein as the “airgap height.” In one embodiment, the air gap height is greater than 0 cm.In one embodiment, the air gap height is greater than 0.1 cm. In oneembodiment, the air gap height is greater than 1 cm. In one embodiment,the air gap height is greater than 5 cm. In one embodiment, the air gapheight is greater than 10 cm. In one embodiment, the air gap height isgreater than 20 cm. Larger air gap heights favor skin formation.

Similarly, relatively high spinning dope temperatures (i.e., thetemperature of the spinning dope just before extrusion through thespinneret) favor skin formation. The spinning dope temperature can begreater than about 40° C. Alternatively, the spinning dope temperaturecan be greater than about 50° C. As yet another alternative, thespinning dope temperature can be greater than about 60° C.

As stated above, according to one embodiment, the coagulating bathcontains de-ionized water. A sufficiently high coagulating bathtemperature ensures adequate phase separation in the coagulating bath.If phase separation is inadequate, the fibers will be crushed in thefirst guide roll after extrusion. The coagulating bath temperature canbe between about 10° C. and about 70° C. Alternatively, the coagulatingbath temperature can be between about 25° C. and about 60° C. As anotheralternative, the coagulating bath temperature can be between about 40°C. and about 50° C.

The take-up rate, i.e., the speed at which the hollow fibers are woundaround the take-up drum, can be much greater than take-up rates usedwhen spinning low molecular weight polymers. This is due to the factthat the high molecular weight polymers as described herein canwithstand the greater stresses associated with higher take-up rates. Thetake-up rate can be increased with a fixed extrusion rate if a smallerdiameter fiber is required. Take-up rates between about 20 m/min andabout 150 m/min are achievable according to the method as describedherein.

The face velocity of air surrounding the spinneret can be greater than50 ft/min (15 m/min). Alternatively, the face velocity of airsurrounding the spinneret can be greater than 80 ft/min (24 m/min). Asanother alternative, the face velocity of air surrounding the spinneretcan be greater than 100 ft/min (30 m/min).

Transesterification Reaction

The transesterification reaction involves subjecting the monoesterifiedpolyimide polymer to transesterification conditions to form acrosslinked membrane. FIG. 2 schematically illustrates thetransesterification reaction. In the transesterification reaction, theOH groups in esters in one monoesterified polyimide polymer chain reactwith esters in another monoesterified polyimide polymer chain to form atransester or crosslink. Any unconverted —COOH groups in onemonoesterified polyimide polymer chain can also react with —OH groups inesters in another monoesterified polyimide polymer chain to form acrosslink. In this manner, the transesterification reaction crosslinksthe monoesterified polyimide polymer chains.

The crosslinked hollow fiber membrane module is comprised of individualfibers of crosslinked polyimide polymer chains. For example, thecrosslinked hollow fiber membrane can comprise an array of such fibers.

The crosslinked membrane is suitable for separating fluid mixtures,including both gaseous mixtures and liquid mixtures. The crosslinkedhollow fiber membrane exhibits better permeability and selectivity thancrosslinked hollow fiber membranes made from low molecular weight,monoesterified polyimide polymers.

Transesterification Conditions

Typical transesterification conditions are known in the art. Generally,transesterification can be accomplished by heating the monoesterifiedpolyimide polymer. Heating initiates the transesterification reactionand, additionally, removes residual solvent.

The monoesterified, polyimide polymer can be heated to crosslink at atemperature of about 150° C. or higher under vacuum. In one embodiment,the monoesterified, polyimide polymer is heated to crosslink at atemperature of about 180° C. or higher under vacuum. In anotherembodiment, the monoesterified, polyimide polymer is heated to crosslinkat a temperature of about 200° C. or higher under vacuum. For example,the monoesterified hollow fibers can be heated under vacuum at 200° C.for approximately 2 hours and cooled under vacuum for approximately 6hours. Higher temperatures result in a greater degree of crosslinking.However, temperatures of about 300° C. or higher may damage the skinlayer of a crosslinked hollow fiber membrane made according to themethods as described herein.

Transesterification can also be accomplished by UV or microwavetreatment. Furthermore, transesterification reactions can be catalyzed.Transesterification catalysts can be the same acid catalysts used duringmonoesterification, which include para-toluene sulfonic acid, sulfuricacid, methanesulfonic acid, triflic acid, and mixtures thereof.

Separation Systems Including the Membranes

Membranes as disclosed herein can be used in separation systems likethose discussed in U.S. Pat. Nos. 6,932,859 and 7,247,191, which areincorporated herein by reference in their entirety.

The membranes made from the high molecular weight, monoesterifiedpolyimide polymer may take any form known in the art, for example,hollow fibers, tubular shapes, and other membrane shapes. Other membraneshapes include spiral wound membranes, pleated membranes, flat sheetmembranes, and polygonal membranes.

Hollow fibers as described herein can be employed in bundled arraysembedded in a sealant (potted) at either end to form tube sheets andfitted into a pressure vessel thereby isolating the insides of the tubesfrom the outsides of the tubes. The fibers are held together by anyconventional means. Typically one end of the fiber bundle extends to oneend of the pressure shell and the opposite end of the fiber bundleextends to the opposite end of the pressure shell. The fiber bundle isfixably or removably affixed to the pressure shell by any conventionalmethod to form a pressure tight seal. Devices of this type are known inthe art. In separation systems of this type, the direction of flow in ahollow fiber element can be counter-current rather than co-current oreven transverse.

Such counter-current flow can be achieved by wrapping the hollow fiberbundle in a spiral wrap of flow-impeding material. This spiral wrapextends from a central mandrel at the center of the bundle and spiralsoutward to the outer periphery of the bundle. The spiral wrap containsholes along the top and bottom ends whereby gas entering the bundle fortube side flow at one end is partitioned by passage through the holesand forced to flow parallel to the hollow fiber down the channel createdby the spiral wrap. This flow direction is counter-current to thedirection of flow inside the hollow fiber. At the bottom of the channelsthe gas re-emerges from the hollow fiber bundle through the holes at theopposite end of the spiral wrap and is directed out of the module.

Industrial hollow fiber membrane modules typically contain hundreds ofthousands of individual hollow fibers. The number of fibers bundledtogether will depend on fiber diameters, lengths, and porosities and ondesired throughput, equipment costs, and other engineeringconsiderations understood by those in the chemical engineering arts.

Specifically, to maximize productivity, the hollow fibers typicallyinclude a “skin” layer on a porous support. Generally, the thickness ofthe skin layer can range from about 0.025 microns to about 1 micron. Gasseparation is accomplished through this selective “skin.” This outer“skin” layer may be supported on the same polymer to form an integrallyskinned asymmetric hollow fiber membrane. The most advanced membraneshave an asymmetric sheath with the selective skin supported on aninexpensive porous core layer (different polymer) to form a compositehollow fiber membrane. This type of device is described in U.S. Pat. No.5,085,676, the contents of which are incorporated by reference herein intheir entirety.

Sheets can be used to fabricate a flat stack permeator that includes amultitude of membrane layers alternately separated by feed-retentatespacers and permeate spacers. The layers can be glued along their edgesto define separate feed-retentate zones and permeate zones. Devices ofthis type are described in U.S. Pat. No. 5,104,532, the contents ofwhich are herein incorporated by reference in their entirety.

The membranes can be included in a separation system that includes anouter perforated shell surrounding one or more inner tubes that containmembranes. The shell and the inner tubes can be surrounded with packingto isolate a contaminant zone.

In one mode of operation, a gaseous mixture enters the separation systemvia a contaminant collection zone through the perforations in the outerperforated shell. The gaseous mixture passes upward through the innertubes.

As the gaseous mixture passes through the inner tubes, one or morecomponents of the mixture permeate out of the inner tubes through theselective membrane and enter the contaminant collection zone.

The membranes can be included in a cartridge and used for permeatingcontaminants from a gaseous mixture. The contaminants can permeate outthrough the membrane, while the desired components continue out the topof the membrane. The membranes can be stacked within a perforated tubeto form the inner tubes or can be interconnected to form aself-supporting tube.

Each one of the stacked membrane elements can be designed to permeateone or more components of the gaseous mixture. For example, one membranecan be designed for removing carbon dioxide, a second for removinghydrogen sulfide, and a third for removing nitrogen. The membranes canbe stacked in different arrangements to remove various components fromthe gaseous mixture in different orders.

Different components can be removed into a single contaminant collectionzone and disposed of together, or they can be removed into differentzones. The membranes can be arranged in series or parallelconfigurations or in combinations thereof depending on the particularapplication.

The membranes can be removable and replaceable by conventional retrievaltechnology such as wire line, coil tubing, or pumping. In addition toreplacement, the membrane elements can be cleaned in place by pumpinggas, liquid detergent, or other material past the membrane to removematerials accumulated on the membrane surface.

A gas separation system including the membranes described herein can beof a variable length depending on the particular application.

The gaseous mixture can flow through the membrane(s) following aninside-out flow path where the mixture flows into the inside of thetube(s) of the membranes and the components which are removed permeateout through the tube. Alternatively, the gaseous mixture can flowthrough the membrane following an outside-in flow path.

In order to prevent or reduce possibly damaging contact between liquidor particulate contaminates and the membranes, the flowing gaseousmixture can be caused to rotate or swirl within an outer tube. Thisrotation can be achieved in any known manner, for example, using one ormore spiral deflectors. A vent can also be provided for removing and/orsampling components removed from the gaseous mixture.

Ideally, the membranes are durable, resistant to high temperatures, andresistant to exposure to liquids. The materials can be coated, ideallywith a polymer, to help prevent fouling and improve durability. Examplesof suitable polymers include those described in U.S. Pat. Nos. 5,288,304and 4,728,345, the contents of which are incorporated by referenceherein in their entirety. Barrier materials can also be used as apre-filter for removing particulates and other contaminants which candamage the membranes.

It will be understood that various modifications can be made to theembodiments disclosed herein. Therefore the above description should notbe construed as limiting, but merely as exemplifications of preferredembodiments. For example, the functions described above and implementedas the best mode for operating the present invention are forillustration purposes only. Other arrangements and methods may beimplemented by those skilled in the art without departing from the scopeand spirit of this invention. Moreover, those skilled in the art willenvision other modifications within the scope and spirit of the claimsappended hereto.

What is claimed is:
 1. A method for preparing a crosslinked hollow fibermembrane, the method comprising spinning into a monoesterifed fiber aone phase solution comprising a monoesterified polyimide polymer,acetone as a volatile solvent, a spinning solvent, and a spinningnon-solvent, wherein the volatile solvent is present in an amount ofgreater than 25 wt. % to about 50 wt. %, based on the total weight ofthe solution; and subjecting the monoesterified fiber totransesterification conditions to prepare a crosslinked hollow fibermembrane.
 2. The method of claim 1, wherein the monoesterified polyimidepolymer is present in the spinning solution in an amount between about20 and about 50 weight percent.
 3. The method of claim 1, wherein themonoesterified, polyimide polymer has an average molecular weightbetween about 50,000 and about 300,000.
 4. The method of claim 1,wherein the monoesterified, polyimide polymer has a polydispersity indexbetween about 2 and about
 5. 5. The method of claim 1, wherein thevolatile solvent is present in an amount of greater than 25 wt. % toabout 35 wt. %, based on the total weight of the solution.
 6. The methodof claim 1, wherein the volatile solvent is present in an amount ofabout 35 wt. % to about 50 wt. %, based on the total weight of thesolution.
 7. The method of claim 1, wherein the spinning solvent is anorganic solvent selected from the group consisting ofN-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO),dimethylformamide (DMF), dimethylacetamide (DMAc), and diglyme.
 8. Themethod of claim 1, wherein the one phase solution further comprises oneor more inorganic additives.
 9. The method of claim 8, wherein theinorganic additive is an antilyotropic salt.
 10. The method of claim 1,wherein the spinning non-solvent is selected from the group consistingof a C₂ to C₁₀ aliphatic alcohol, water, and mixtures thereof.
 11. Themethod of claim 1, wherein the spinning non-solvent is ethanol.
 12. Themethod of claim 1, wherein the monoesterified hollow fiber is spun fromthe spinning dope by a dry-jet/wet-quench spinning process comprising:(a) extruding the monoesterified polyimide polymer through orifices of aspinneret to provide a monoesterified hollow fiber; (b) conveying themonoesterified hollow fiber through an air gap and through a coagulatingbath of de-ionized water; and (c) winding the monoesterified hollowfiber around a take-up drum at a take-up rate between about 10 m/min andabout 150 m/min, wherein the dry-jet/wet-quench spinning process has adraw ratio of less than
 150. 13. The method of claim 12, wherein thetemperature of the spinning solution is greater than about 40° C. 14.The method of claim 12, wherein the temperature of the coagulating bathis between about 10° C. and about 70° C.
 15. The method of claim 12,wherein the dry-jet/wet-quench spinning process has an air gap heightthat is greater than 0.1 centimeter.
 16. The method of claim 12, whereinthe dry-jet/wet-quench spinning process has a face velocity of airsurrounding the spinneret that is greater than 15 m per minute.
 17. Themethod of claim 1, wherein the resulting crosslinked membrane comprisesa porous substructure and a relatively defect free dense skin of uniformdensity.
 18. A method of using a hollow fibers membrane made accordingto the method of claim 1, comprising: (a) providing a feed streamselected from the group consisting of air, a mixture of methane andnitrogen, a mixture of methane and hydrogen, a mixture of methane andhydrogen sulfide, a refinery stream, a mixture of carbon dioxide andmethane, and syngas, the feed stream including a gaseous componentselected from the group consisting of nitrogen, oxygen, hydrogen,hydrogen sulfide and carbon dioxide; (b) maintaining a pressuredifferential between an upstream side of the membrane and a downstreamside of the membrane; (c) contacting the upstream side of the membranewith the feed stream at a pressure between about 20 psia and about 4000psia; (d) isolating a permeate stream on the downstream side of themembrane having a larger mole fraction of the faster permeatingcomponent of the feed stream; and (e) isolating a retentate streamhaving a smaller mole fraction of the faster permeating component of thefeed stream.
 19. A method for preparing a crosslinked hollow fibermembrane, the method comprising spinning a one phase solution comprisinga monoesterified polyimide polymer, a volatile solvent having athreshold limit value time-weighted average (TLV-TWA) toxicity ofgreater than 200 parts per million (ppm) exposure limit, a spinningsolvent, and a spinning non-solvent, wherein the volatile solvent ispresent in an amount of greater than 25 wt. % to about 50 wt. %, basedon the total weight of the solution.